Add API client for Azure custom vnet (#271)

* Update vendor for azure vent support

* Add support for Azure custom vnets.

Use pointers intead of values. This allows the client to pass back
returned data from Azure.

providers/azure/client/network/client.go

@@ -0,0 +1,63 @@
+package network
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/Azure/azure-sdk-for-go/services/network/mgmt/2018-05-01/network"
+	"github.com/Azure/go-autorest/autorest/azure/auth"
+	azure "github.com/virtual-kubelet/virtual-kubelet/providers/azure/client"
+	"github.com/virtual-kubelet/virtual-kubelet/providers/azure/client/api"
+)
+
+const (
+	baseURI    = "https://management.azure.com"
+	userAgent  = "virtual-kubelet/azure-arm-networking/2018-05-01"
+	apiVersion = "2018-05-01"
+)
+
+// Client is a client for interacting with Azure networking
+type Client struct {
+	sc network.SubnetsClient
+	hc *http.Client
+
+	auth *azure.Authentication
+}
+
+// NewClient creates a new client for interacting with azure networking
+func NewClient(azAuth *azure.Authentication) (*Client, error) {
+	if azAuth == nil {
+		return nil, fmt.Errorf("Authentication is not supplied for the Azure client")
+	}
+
+	client, err := azure.NewClient(azAuth, baseURI, userAgent)
+	if err != nil {
+		return nil, fmt.Errorf("Creating Azure client failed: %v", err)
+	}
+
+	authorizer, err := auth.NewClientCredentialsConfig(azAuth.ClientID, azAuth.ClientSecret, azAuth.TenantID).Authorizer()
+	if err != nil {
+		return nil, err
+	}
+
+	sc := network.NewSubnetsClient(azAuth.SubscriptionID)
+	sc.Authorizer = authorizer
+
+	return &Client{
+		sc:   sc,
+		hc:   client.HTTPClient,
+		auth: azAuth,
+	}, nil
+}
+
+// IsNotFound determines if the passed in error is a not found error from the API.
+func IsNotFound(err error) bool {
+	switch e := err.(type) {
+	case nil:
+		return false
+	case *api.Error:
+		return e.StatusCode == http.StatusNotFound
+	default:
+		return false
+	}
+}